As is well known, a digital signal transmitted over a digital line is affected by noise or the like on a transmission path to experience fluctuating phase.
In the fluctuations of the phase, components in the fluctuations at frequencies higher than 10 Hz are referred to as jitter, and components lower than 10 Hz as wander.
As such phase fluctuations become larger, the line cannot correctly transmit a digital signal thereon, resulting in larger code errors.
It is therefore necessary to measure jitter and wander for evaluating a digital line.
Among them, as an evaluation method associated with the wander, a time deviation (hereinafter designated as TDEV) is known.
The measurement of TDEV involves sequentially finding a phase difference TIE (Time Interval Error) between a clock signal component of a digital signal including wander and a reference clock signal as a changing amount with respect to its initial phase difference, and calculating the following equation based on this TIE data.TDEV(τ)={(1/6n2)(1/m)·j=1Σm[i=jΣn+j−1(xi+2n−2xi−n+xi)]2}1/2 where m=N−3n+1; xi is TIE sample data; N is the total number of samples, τ is an integration time (τ=n·τ0), n is a sampling number (n−1, 2, . . . , N/3), τ0 is a sampling period, a symbol j=1Σm is a sum of j=1−m; and a symbol i=jΣn+j−1 is a sum of i=j−n+j−1.
TDEV(τ) is found based on all TIE data over a measuring time 12 times a maximum integration time.
For example, for finding TDEV(1000) for τ=1000 seconds when the sampling period τ0 is 1/80 seconds (12.5 milliseconds), the above equation is solved using measurement data over 12000 seconds (80 samples/second×1000 seconds×12=960000 samples).
For evaluating a digital line using this TDEV, there is known a method which involves inputting a digital signal without phase fluctuations at one terminal of a line under testing and measuring the TDEV at the other terminal.
Also, there is another method which involves inputting a digital signal synchronized with a clock signal having wander to a line under testing, measuring an error rate of the digital signal at the other end, while changing the magnitude and frequency of the wander, and investigating the tolerance of the line against the magnitude and frequency of the wander.
For evaluating a line under testing using a digital signal including wander, as in the latter method, a wander generator is used for generating a clock signal having phase fluctuations at 10 Hz or lower.
FIG. 50 is a block diagram illustrating the configuration of a conventional wander generator 10.
In this wander generator 10, a modulation signal for modulating a phase lower than 10 Hz output from a modulation signal generator 11 and a reference voltage output from a reference voltage generator are added by an adder 13.
Then, in this wander generator 10, the output of the adder 13 is input to a VCO (voltage controlled oscillator) 14 to generate a clock signal CK which has a center frequency corresponding to the reference voltage and phase modulated by the modulation signal.
This wander generator 10 can vary the frequency and magnitude of the clock signal CK by varying the frequency and amplitude of the modulation signal output from the modulation signal generator 11.
In recent years, there has been proposed a method which evaluates a digital line using a digital signal that has wander which is referred to a TDEV mask (Mask) and satisfies a TDEV characteristic defined by ANSI (American National Standards Institute) or the like.
The TDEV mask has a characteristic M1 (Section 7.22 in ANSI T1.101-1994, Section D.2.2.1 in 105-03-1994, and the like) which is constant until a certain integration time τ1, and increases in proportion to τ1/2 in a range exceeding the integration time τ1, as illustrated in FIG. 51A.
Also, this TDEV mask has a characteristic M2 (Section 7.3.2 in ANSI T1. 101-1994, Section D.2.1 and Section D.2.2.2 in 105-03-1994) which is constant until a certain integration time τ1, increases in proportion to τ1 in a range of the integration time from τ1 to τ2, and increases in proportion to τ1/2 in a range exceeding the integration time τ2, as illustrated in FIG. 51B, and the like.
However, since the conventional wander generator 10 as described above can only phase modulate a single signal, it encounters difficulties in generating a clock signal which satisfies the TDEV characteristic that varies in each integration time range as described above.
It is therefore desired to realize in this type of field a wander generator which is capable of generating a clock signal having wander of desired characteristic that satisfies an arbitrary TDEV mask characteristic, and a digital line tester using this wander generator.
As described above, a transmission system for transmitting a clock and data cannot correctly restore data if a transmitted signal has larger phase noise (phase fluctuations).
For this reason, it is necessary to examine a transfer characteristic for a signal having phase noise for manufacturing or maintaining devices for use in this type of transmission system.
As mentioned above, in the fluctuations of the phase, components in the fluctuations at frequencies higher than 10 Hz are referred to as jitter, and components lower than 10 Hz as wander.
As such, jitter and wander are collectively referred herein to as phase noise.
Also, here, the phase noise is not a periodic function signal such as a single sinusoidal signal or the like which has a constant frequency and amplitude, but a noise signal which has a frequency characteristic over a wide band.
Generally, the characteristics of phase noise are represented by:
(a) TDEV (Time DEViation);
(b) TIErms (Root Mean Square Time Interval Error);
(c) MADEV (Modified Allan DEViation);
(d) ADEV (Allan DEViation); and the like.
In recent years, these characteristics are being standardized.
Therefore, for evaluating the phase noise transfer characteristic for a device, it is necessary to use a test signal having jitter and wander that conform to these standardized characteristics.
Specifically, it is necessary to input a test signal having jitter and wander of predetermined characteristics to a device under analysis, and examine how a phase noise characteristic resulting from a measurement of the phase noise characteristic of the output changes with respect to the standardized characteristics.
For analyzing such phase noise transfer characteristic, a phase noise transfer characteristic analyzer 100 as illustrated in FIG. 52 has been conventionally used.
This phase noise transfer characteristic analyzer 100 comprises characteristic specifying means 111 for specifying an arbitrary phase noise characteristic including the aforementioned standardized characteristic; parameter calculating means 112 for calculating parameters required to generate a test signal having the specified phase noise characteristic; test signal generating means 113 for generating a test signal having a phase noise characteristic corresponding to the calculated parameters and outputting the test signal from an output terminal 100a; phase noise characteristic measuring means 114 which receives through an input terminal 100a an output signal of a device 1 under analysis which has received the test signal output from the output terminal 100a for measuring its phase noise characteristic; and display means 115 for displaying the phase noise characteristic specified by the characteristic specifying means 111 and the phase noise characteristic measured by the phase noise characteristic measuring means 114 in such a manner that they can be compared with each other.
Next, description is made on an analysis made on a transfer characteristic for TDEV of wander using the phase noise transfer characteristic analyzer 100.
For example, as illustrated in FIG. 53, as a characteristic R of TDEV which has a slope which changes on boundaries located at integration times τ1 and τ2 is specified by the characteristic specifying means 111, the parameter setting means 12 calculates parameters corresponding to the characteristic R for setting in the test signal generating means 113.
Then, the test signal generating means 113 generates a test signal St with a phase noise characteristic determined by the parameters, and outputs the test signal St to the device 1 under analysis through the output terminal 100a. 
An output signal Sr of the device 1 under analysis, which has received the test signal St, is input to the phase noise characteristic measuring means 114 through the input terminal 100b to measure a characteristic M of TDEV of the signal Sr.
Then, as illustrated in FIG. 54, the characteristic R specified by the characteristic specifying means 111 and the characteristic M measured by the phase noise characteristic measuring means 114 are displayed on the display means 115.
It is therefore possible to evaluate the wander transfer characteristic of the device 1 under analysis by comparing the two characteristics, displayed on the display means 115, with each other.
However, in this case, the phase noise characteristic of the test signal St input to the device 1 under analysis cannot actually be matched completely with the characteristic R specified by the characteristic specifying means 111.
Specifically, as illustrated in FIG. 53, the characteristic R generally used for evaluating phase noise is a theoretical characteristic, the slope of which is indicated by a folded line which discontinuously varies.
It is therefore extremely difficult to realize such a theoretical characteristic with an actual electronic circuit.
For this reason, the test signal St actually output from the test signal generating means 111 has a characteristic which has the slope changing portions of the characteristic R approximated by curves as R′ in FIG. 53.
As such, for comparing the characteristics displayed on the display means 15, the operator himself must make an analysis in consideration of an error in the characteristic due to the approximation, so that a precise comparison is extremely difficult to achieve.
To solve this problem, the output terminal 100a has been previously connected directly to the input terminal 100b, as indicated by a broken line in FIG. 52, to measure the phase noise characteristic of the test signal St by the phase noise characteristic measuring means 114.
Then, it is contemplated that an approximation error has been found for the phase noise characteristic of the test signal St and a characteristic specified by the characteristic specifying means 112, such that a phase noise characteristic derived when the device 1 under analysis is measured is corrected by the approximation error.
However, such a method of finding the phase noise characteristic of the device 1 under analysis after finding the phase noise characteristic of the test signal requires a double measuring time, so that a waiting time until the result of a measurement is output becomes very long, particularly, for an analysis of a transfer characteristic for wander which requires a long measuring time.